Electron gun power regulation method and apparatus



Dec. 8, 1970 ANDERSON ETAl. 3,546,606

ELECTRON GUN POWER REGULATION METHOD AND APPARATUS Filed Oct. 13, .1969 4 Sheets-Sheet 1 COMPARISON NETWORK Emmi/WM imiika fun ATTORNEY5 Dec. 8; 197.0 ANDERSON ETIAl. 3,546,606

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Dee. 8,, 1970 E. R. AND ER$ON E A ELECTRON GUN POWER REGULATION METHOD AND APPARATUS 4 Sheets-Sheet 3 Filed Oct. 13,- 1969 FIG. 3.

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. EMMETTR. ANDERSON CLIFFORD G. WALLACE W 71% ,am y P ial; ATTORNEYS United States Patent Oflice US. Cl. 328-267 15 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus are described for regulating the power of an electron gun employed in an electron beam furnace. A rise in current to the electron gun due to an arc is sensed and gun current is cut back quickly in a manner which starves any arcs in their incipiency. As a result, power may be rapidly restored to the electron gun without coincident restoration of arcs.

The present invention relates generally to electron beam furnaces and, more particularly, to an improved method and apparatus for regulating the power of one or more electron beam guns employed in a high vacuum electron beam furnace. This application is a continuationin-part of co-pending application, Ser. No. 642,626, filed May 3, 1967, which is a continuation-inpart of application, Ser. No. 546,651, filed May 2, 1966, now abandoned.

The employment of electron beam furnace systems in various material treating processes such as melting, vapor plating, etc., has become increasingly prevalent. A typical electron beam furnace system includes an electron gun, which is appropriately energized to furnish a high intensity beam of electrons. The electron gun is generally disposed in an evacuated chamber together with the material to be treated, and means are provided for directing the electron beam at the material. The electron gun usually includes a source of electrons such as a heated cathode or filament and an accelerating anode, maintained at a high positive potential with respect to the cathode so as to establish a high electrostatic field for accelerating the electrons. A suitable magnet is also usually provided for directing the electrons onto the target material. As the beam of electrons impinges on the target material, the material is heated with the amount of heat developed being related to the electron beam current and the electron velocity effected by the accelerating electrostatic field through which the electrons are directed.

During bombardment of the target material by the electron beam, various vaporous materials are emitted and, in addition, various occluded gases may be released, particularly when the target material is in a relatively impure condition. The presence of such gaseous materials often effects a substantial decrease in the resistance between various parts of the electron beam gun and leads and surrounding elements. This may result in arcing between such parts and leads and elements, causing a substantial increase in the electron gun current and possibly resulting in harm to the electron gun structure and sur rounding elements. To minimize the harmful effects of arcing, various voltage and current regulated electron gun power supplies have been developed.

Some previously known techniques for regulating gun power in electron beam furnace systems have limited the deterimental effects of arcing by limiting current. By limiting the current rise in the presence of an arc to a maximum predetermined value, usually less than about 3,546,606 Patented Dec. 8, 1970 twice the normal operating current, the arc will often quickly terminate and normal operation may be resumed. Current limiting power regulation circuits employing voltage regulation, as shown in US. Pats. Nos. 3,328,672 and 3,400,207, have been successfully used in many instances. Some current limiting regulation circuits, however, are of the monocyclic variety and require extensive protective circuits to preclude over-stressing of the various network elements and also require very large reactive elements.

For power levels, in the range of several KW operating with two or more amperes of beam current, arcs tend to be self-sustaining. Accordingly, it is sometimes impossible to eliminate an existing arc unless the electron gun current is cut back or turned off. Moreover, unless the current is cut back or turned off for a significant length of time, i.e. of the order of half a second, arcs may be restored coincident with restoration of electron gun current. In order to provide for out back or turn off of electron gun current, prior art devices have been designed which place a tiode in series with the electron gun and its DC power supply, and which use a sensing circuit for sensing a rise in current to the gun above a predetermined level. The sensing circuit applies bias to the grid of the triode to cut off gun current when such a current rise occurs. After a period of delay, electron gun current is restored by removing the bias on the grid of the triode.

Prior art systems have suffered from two major drawbacks. First of all, prior art circuits have been able to incorporate only a single electron beam gun for each DC. power supply. Accordingly, such systems are not adaptable to large furnace operations where a plurality of electron guns are employed, since a separate and usually expensive D.C. power supply is required for each gun.

An additional handicap suffered by prior art systems is that they were designed for use in connection with guns operable at power levels of only several kilowatts at about one or two amperes of beam current. Modern industrial practices may require much higher power levels, for example, power levels of 50 to 200 kilowatts at two or more amperes of beam current. In order to provide such high power levels, direct current supplies must typically be made with very low impedance so that they are stiff. This means that for a given voltage level, the rise in beam current will typically exceed five times the operating level within about microseconds. This naturally results in a very high amount of energy being expended in the arc and may have a highly deleterious effect on efficiency. Moreover, the resultant high energy supplied to the arc may require current cut back or turn off periods of excessive length, in order that ionized particles and localized heat in thermionically emissive surfaces may dissipate. This, of course, also deterimentally affects furnace efficiency and in addition sometimes causes undesirable variation in the temperature of the material being heated.

An object of the invention is to provide an improved method and apparatus for regulating the power of an electron gun employed in an electron beam furnace system.

It is also an object of the present invention to provide a method and apparatus for efficiently operating at relatively high levels of power one or more electron beam guns employed in a high vacuum electron beam surface system.

It is another object of the invention to provide an improved method and apparatus, for use in an electron beam furnace system, which regulates the voltage applied to the electron beam gun and which regulates the electron beam current.

Another object is the provision of a method and apparatus for regulating the power supplied to an electron beam gun which automatically and substantially reduce 3 the electron gun current upon the occurrence of arcing in the electron gun.

A further object is to provide such a method and apparatus, particularly suited to operation at power levels exceeding about 50 kw., which permits the electron gun current to be rapidly restored without coincident restoration of arcs.

Still another object is the provision of electron gun power regulating apparatus which is adaptable for systems supplying power to more than one electron beam gun and which cuts back the current supplied to a particular gun during arcing across the gun without having an affect on the operation of the other guns.

A further object of the present invention is the provision of electron gun power regulating apparatus which is relatively simple to operate, durable in use, and relatively high in efficiency.

Other objects of the invention will become readily apparent from the following detailed description and the accompanying drawings wherein:

FIG. 1 is a schematic circuit diagram, in which some of the elements are shown in block form, of one embodiment of apparatus embodying the invention;

FIG. 2 is a schematic circuit diagram of forms of sensing and switching networks adaptable for use in the circuit shown in FIG. 1;

FIG. 3 is a graph illustrating the voltage vs. current characteristics of the apparatus of FIG. 1;

FIG. 4 is a graph illustrating a typical variation in current with time, under arcing conditions, of the apparatus of FIG. 1; and

FIG. 5 is a block diagram of another embodiment of the invention.

Generally, the illustrated apparatus provides direct current from a power supply .10 to an electron gun 12 in an electron beam furnace 14. The current supplied to the electron gun 12 is sensed by a current sensing means 24. If the current rises to a preselected level (indicating that an arc is developing), an electrical signal is caused to be coupled to a current control device 16, which cuts back the current through the current control device. Current to the electron gun 12 is thereby cut back to reduce the undesired effects of arcing. By providing for extremely rapid current cut back, preferably less than a millisecond from the beginning of development of an arcing condition, the incipient arc is starved and normal operation may be restored very quickly, for example, in less than one or two milliseconds.

For the sake of simplicity, the present system is described herein as employing only a single electron gun. However, it may be readily seen that the apparatus is particularly adaptable for energizing a plurality of electron guns from a single power supply by merely coupling additional current control means and the associated control circuits in parallel relationship with the apparatus shown, using common output terminals on the power supply 10. This results from the particular circuit configuration illustrated in which one end of the power supply is grounded as is the electron gun 12, thereby providing a common ground for a plurality of electrically parallel electron guns coupled through their associated control circuits to a common output terminal on the power supply 10. It is to be understood, however, that the method of the invention in its broadest sense is applicable to other systems, for example, a system wherein the positive side of the electron gun is not directly connected to a common ground but is connected to the positive side of the power supply 10 through the current control device 16.

More specifically, referring to FIG. 1 of the drawings, the power supply 10 is a relatively high voltage direct current power supply adaptable for providing a desired voltage and a desired current. For high power applications, the power supply is typically still as explained more fully below. Obviously, if two electron guns were being employed, the current required from the section 28 of the power supply 10, would be doubled, while if three guns were being utilized the current would be tripled, etc. The power supply is preferably provided with an input transformer having high voltage insulation on its windings so as to suitably isolate the output from the input signal. The power supply section 28 also includes a conventional three-phase, full wave bridge rectifier and an output filter which is designed to provide minimal ripple so as to avoid difficulties in achieving desired focusing of the electron beam. The power supply section 28 is energized by a suitable source of three-phase A-C power (not shown).

In the illustrated embodiment, the output of the power supply section 28 is connected through the current control device 16 to the electron gun 12. In this connection, the positive output terminal of the power supply section 28 is connected to ground, and the negative output terminal is connected to the current control device 16.

The illustrated current control device is a vacuum triode 16, which includes a directly heated cathode 30, an anode 32, and a control element, or grid 20. For high power applications, the triode or equivalent vacuum tube is preferably one with a very low impedance under normal operating conditions, and which is capable of being back biased to a very low level of leakage current, as explained below. The cathode 30 is heated by a suitable filament transformer 34 provided with high voltage insulation on its primary winding 36 and on its secondary winding 38. In this connection, the primary winding 36 of the filament transformer 34 is connected to a suitable source 39 of alternating current. The secondary winding 38 is provided with a center tap 40 and is connected to the cathode 30. In addition, a pair of capacitors 42 and 44 are coupled between the ends of the secondary winding 38 and the center tap 40. The negative terminal of the power supply section 28 is coupled to the center tap 40 of the filament transformer 34 through a current sensing resistor 46 (the purpose of which is described below) and a cathode resistor 48 which provides degeneration for preventing oscillations of the circuit and assisting in turn-01f of the triode.

The anode 32 of the triode 16 is coupled through a conductor 50 to the electron gun 12, which may be of any suitable type. In the illustrated embodiment, the electron gun 12 is disposed in the suitably grounded furnace 14 and includes a directly heated cathode 54, and a grounded accelerating anode 56. The beam of electrons emitted by the cathode 54 is accelerated by the accelerating potential established between the cathode 54 and the grounded anode 56, and is deflected by a transverse field produced by an appropriately positioned electro-magnet 58 onto the surface of a target material 60. The cathode 54 is heated by a suitable filament transformer 62 provided with high voltage insulation on its windings. The primary winding 64 of the filament transformer 62 is connected to the current adjustment network 26 (described more fully below) while its secondary winding 66 is connected to the cathode 54. The secondary winding 66 also includes a center tap 68 which is connected to the conductor 50 and hence to the triode 16. In addition, a pair of capacitors 70 and 72 are coupled between the ends of the secondary winding 66 and the center tap 68.

Accordingly, a complete series circuit is provided extending from the grounded positive terminal of the power supply section 28, through the negative terminal, through the triode 16 and the conductor 50, and through the electron beam emitted by the cathode 54 to the grounded accelerating anode 56 and target material 60. The triode 16 is thus electrically in series relationship with the electron beam current and is controlled to limit the intensity of the electron beam, as described below. When operating in accordance with the method of the invention, the triode 16 may be connected in another series position with the gun, for example, between the positive terminal of the power supply section 28 and the positive side of the gun, with the cathode being at ground potential.

It is generally desirable to prevent the occurrence of parasitic oscillations in the circuit which might interfere with the regulation. This is conveniently achieved by the employment of an inductively wound resistor 76 connected in series with the anode 32 of the triode 16.

The voltage which is developed across the electron gun 12 is generally held constant, or regulated through utilization of the voltage sensing network 18. The voltage regulation function is effected by connecting a voltage divider network 78 across the electron gun 12. The voltage divider 78 includes a first resistor 80, having one end connected intermediate the anode 32 of the triode 16 and the cathode 54 of the electron gun 12. A second resistor 82 is serially connected to the opposite end of the first resistor at one end and is connected to ground at its other end, which is a point of common electrical potential with both the anode 56 of the electron gun 12 and the power supply section 28.

A predetermined proportion of the voltage developed across the electron gun 12, dependent upon the relative values of the resistors 80 and 82, appears across the resistor 82 and is matched against the output voltage of an adjustable reference voltage source 84 by a conventional voltage comparison network 86. The difference voltage is utilized to maintain the desired voltage level across the electron gun 12. Thus, control, or adjustment of the electron gun voltage is obtained through varying the output of the adjustable voltage reference source 84.

The difference voltage signal, which is related to the output signal provided by the triode 16, is fed back to the grid 20 of the triode 16 so as to obtain the desired voltage regulation. In this connection, the difference voltage is amplified by an amplifier 88 connected to the output of the voltage comparison network 86. The amplified signal is converted to an alternating current signal by a conventional DC to A-C converter 90 connected to the output of the amplifier 8 8 and is applied across a primary winding 92 of an isolation transformer 94, provided with high voltage insulation on its windings. The signal produced at the secondary windin 96 of the transformer 94 is supplied to a conventional A-C to D-C converter 98 connected across the secondary winding 96. The resulting DC signal is then supplied through a conductor 100 to a conventional series voltage regulator 102, which has its output coupled to the grid 20 of the triode 16 through the normally closed switch means 22.

The series voltage regulator 102 is energized by connecting its input to a section 104 of the power supply 10 for providing a bias power supply. In this connection, the positive output terminal of the bias supply 104 is connected to the regulator 102 through a conductor 106, while its negative output terminal is coupled to the negative output terminal of the power supply section 28. The bias supply 104 also has its input connected to the three phase A-C source. During normal operation of the electron gun, the positive terminal of the bias supply 104 is coupled to the grid 20 of the triode 16 through the regulator 102 and the normally closed switch means 22. The voltage regulator 102 receives the D-C feedback signal from the converter 98 and regulates the positive voltage supplied by the bias supply 104. Preferably, the series regulator 102 is of the conventional transistor variety provided with suitable clamping and oscillation prevention circuits in order to prevent the occurrence of high current surges resulting from switchin transients generated by triggering of the switch means 22. During normal operation the positive output from the regulator 102 is coupled through the closed switch means 22 to a grid resistor 108 connected to the grid 20 of the triode 16 so as to maintain the triode 16 in a highly conductive state. The grid resistor 108 is preferably positioned relatively close to the grid 20 so as to aid in the suppression of parasitic oscillations in the circuit.

The grid 20 is also coupled to a resistor 110 WhlCh, when current flow through the triode rises due to development of an arcing condition, effects the application of negative bias to the grid 20. The resistor 110 has its output connected to the junction of the output of the switch means 22 and the one end of the grid resistor 108, while its input is connected to the negative output terminal of a further section 112 of the power supply 10 for providing bias power. The bias supply 112 has a positive output terminal, which is connected to the negative output terminal of the power supply section 28 and has its input coupled to the A-C power source. The output from the switch means 22 is of a greater voltage than the voltage at the output end of the resistor 110, and thus when the switch means 22 is closed, positive voltage is applied to the grid 20. But, when an arcing condition occurs, effecting the opening of the switch means 22 as is subsequently explained, this positive bias is no longer applied to the grid 20. Instead, a negative bias is applied from the second bias supply 112 through the resistor 110 rendering the triode 16 substantially less conductive, the current supplied to the electron gun 12 is cut back, which automatically precludes the maintanence of arcing in the electron gun.

Generally, the current sensing resistor 46, which is serially connected to the triode 16, senses or indicates the current passing through it by developing a voltage related to a current. The sensing circuit 24 is connected across the current sensing resistor 46 and thus receives this voltage signal at its input terminals. The sensing circuit 24 is adjusted so as to generate an electrical signal when the voltage across the sensing resitor 46 rises to a preelected level. Such a voltage rise occurs when an arcing condition exists at the electron gun due to an increase in current being drawn through the triode 16. The sensing circuit signal is coupled to the switch means 22 causing the switch means to open, thereby efiecting the removal of the positive bias from the triode 16 and causing a negative bias to be applied to the grid 20, from the bias supply 112. This renders the triode 16 substantially less conductive.

The sensing circuit 24 is suitably energized by a further section of the power supply 10 which is a bias supply 113. In this connection, the negative output terminal of the bias supply 113 is coupled to the negative output terminal of the power supply section 28, while the positive output terminal of the bias supply 113 is connected to the sensing circuit 24 to supply operating power thereto.

The output of the sensing circuit 24 is coupled to a restart circuit 114, as well as being coupled to the switch 22. The restart circuit 114 is adapted for effecting the closing of the switch means 22, after the switch means 22 has been opened by the presence of an excessive voltage across the resistor 46. The restart circuit 114 is triggered by the sensing circuit 24, when the sensing circuit is energized. After a suitable preselected time delay the restart circuit 114 applies a signal pulse to the open switch 22, which effects closing of the switch 22, thereby permitting a positive bias to be reapplied to the grid 20. Thus, the triode 16 is again rendered fully conductive and normal operation of the power supply is resumed.

Relatively high transient currents may occur at the initiation of arcing resulting in the development of voltages across the sensing resistor 46 greatly exceeding what is necessary for proper sensing circuit response. To prevent damage to the sensing circuit 24 from excessive voltage transients, it is desirable to provide a voltage limiting circuit 116 connected across the sensing resistor 46. The limiting circuit 116 includes a plurality of forward biased diodes 118, serially connected with each other across the resistor 46. The diodes 118 are shown as conventional silicon rectifiers, however, Zener diodes may be used if available with adequate current carrying capacity. In the illustrated embodiment, seven diodes are utilized.

The limiting circuit 116 also includes a reverse biased diode 120 connected across the diodes 118. This diode 120 protects the diodes 118 from breakdown in the event of a relatively high reverse transient signal.

At the initiation of an arcing condition at the electron gun, the voltage drop across the electron gun approaches the zero level so that substantially all of the voltage would tend to appear across the triode 16. Such a result is quite undesirable and is generally prevented from materializing by an inductor 121 connected in series between the anode 32 and the cathode 54 of the electron gun. In this connection, the inductor 121 tends to limit the occurrence of abrupt voltage peaks in the circuit by increasing the rise time of the voltage peak, i.e. causing the peak to be smoothed out over a time interval determined by the characteristic of the inductor. As a result, the current limiting circuit has time in which to react so as to reduce the current being supplied to the electron gun 12 and thus stop the arcing.

During normal operation of a high vacuum electron beam furnace system, arcing between various elements of the electron beam gun and leads and various elements of the furnace may periodically occur. Although the precise conditions which produce arcing are not entirely understood, it is believed that local hot spots producing an increase in the level of thermionic emission, and the presence of significant quantities of positive ions in a particular region, may contribute to the occurrence of an arc. After arcing occurs, it has heretofore usually been necessary to substantially reduce the power supplied to the electron beam gun and maintain it at the reduced level for a period of about of a second or more before power can be restored without coincident restoration of the arc. It is believed that this delay allows the large number of ions in the arcing region to dissipate throughout the vacuum furnace and to allow the regions which have been heated to a high temperature and which may have a high level of thermionic emission, to cool down. A delay of A of a second or more is significant and may contribute to a relatively high level of inefficiency in furnace operation and fluctuation in energy delivered to the material being heated. The latter phenomenon can have a particularly deleterious efiect in the case of vapor deposition operations, since it may produce an intolerable variation in the vapor deposition rate. Moreover, in high power level systems, such as those exceeding about 50 kw. per gun, a delay time of only one millisecond before beam current is restored may represent an intolerable level of inefficiency.

An arc may be described generally as having two stages: an incipient stage which is manifested by a rapid rise in current to the electron gun, and a steady state stage in which the current stabilizes at a point where the arc passes the maximum power. The level of current at the steady state stage, and the duration of the incipient stage are generally governed by the internal impedance of the power supply and the impedance characteristics of the current control device 16. By limiting current, through the use of constant current circuitry, to a level below the higher current steady state stage, damage may be prevented, but the arc continues at the lower current level and may rise to the higher current level steady state stage when current limiting is removed.

In accordance with the method of the invention, the arc is starved in its incipient stage, by cutting back the electron gun current sufficiently. Accordingly, full operating current may be restored very quickly without coincident restoration of the arc. Although not entirely understood, it is believed that fast restoration is possible because extensive ionization of vapor particles in the region of the arc is avoided, or because extensive local superheating of electron emissive surfaces does not occur, or both.

In order to gain the benefit of fast turn-on, as has been previously mentioned, electron gun current is cut back while the arc is in its incipiency. Just how far ahead of the steady state condition, in time, the cutback of current should occur depends upon the particular circuit characteristics and component values, the degree of vacuum in the electron beam furnace, the amount and kind of vapors present around the electron gun, and the particular geometry of the electron gun itself and the surrounding furnace structure. With furnaces of power levels exceeding 10 kw. and two amperes, if the electron gun current is cut back less than about one or two milliseconds after the beginning of an arc, it is usually possible to restore electron gun current within a few hundred microseconds without coincident restoration of the are.

Where a particular system is soft, the benefits of fast turn-01f may be negligible, especially if the power level is only a few kilowatts. This is because the internal impedance of either the D-C power supply or the triode may limit the rate of current rise and the maximum current during arcing to such an extent as to make efliciency losses tolerable. For very high power levels, however, it is usually necessary to employ a power supply and a current control device (under normal operating conditions) of very low internal impedance, causing the system to be relatively stiff. Under such circumstances, current during arcing can rise very rapidly and to very high levels. For purposes of this application, a stiff system may be considered one in which current is capable of rising to at least three times the operating level in 1 millisecond or less. Generally, in such a system, the non-reactive internal impedance provides less than about 30% regulation between no load and full load conditions. For low internal impedance during normal operating conditions, the triode or equivalent vacuum tube typically has a very hard vacuum.

Experience indicates that, for most furnace system configurations operating in excess of 30 kw. beam power per gun and at about five amperes beam current, the electron gun current should be cut back to a minimum current level in order to starve the arc, The level required for satisfactory operation is usually less than 20% of the operating current, and for high reliability it is preferable that it be cut back to less than 10%. Highly satisfactory results are obtained with an operating current of five amperes at 50 kw. or over when the current is cut back to less than one ampere. In the latter case, and with a cutback time approaching 10 microseconds after sensing a current rise, it is usually possible to restore electron gun current very quickly, in some cases as quickly as microseconds from cutback, without coincident restoration of an arc.

A further advantage accrues from rapid cut-off of electron gun current at the incipiency of an arc. This advantage stems from the fact that the presence of an arc is usually accompanied by a high level of radio frequency (RF) transients. The power supply circuitry may be sensitive to such transients and complications may develop during their presence. RF traps may be included in the circuitry at suitable locations to cut down the effect of the RF transients, but this naturally leads to an increase in the cost of the circuit. Because of the reduction in RF transients, flowing from the fact that the arcs are starved in their incipiency, circuit design is simplified in this respect.

As previously mentioned, the electron beam current emitted by the electron gun 12 is adjusted by utilization of the current adjustment network 26. In this connection, the current supplied to the electron gun 12 is preferably sensed by a transductor sensor 122, which is serially coupled between the triode 16 and the electron gun 12. The transductor 122 may comprise a magnetic amplifier or a saturable reactor type device, A satisfactory saturable reactor type device amplifier may include a single conductor extending through a pair of toroidal cores each with an A-C winding, such windings being connected in opposition.

In the current adjustment network 26, the output from the A-C winding 126 of the magnetic amplifier 122 is compared with the output of an adjustable current reference means 128 by a conventional current comparison network 130. The difference between these two signals is amplified by a conventional amplifier 132. The output of the amplifier 13-2 is then applied to the input terminals of a filament current controller 134. The filament current controller 132 generally includes an A-C power source (not shown) the output of which is coupled through a conventional, silicon controlled rectifier gate circuit (not shown) to the primary 64 of the filament transformer 62. The silicon controlled rectifier is fired, or energized, when there is a sufficient difference between the signal provided by the reference means 128 and the current sensed by the magnetic amplifier 122. Such a condition causes a change in the current supplied to the primary winding 64 of the gun filament transformer 62, which controls the heating of the cathode 54. Thus, the current supplied to the electron gun is maintained at a desired level. The desired level of heating of the cathode 54 is conveniently adjusted by merely varying the output of the current reference means 128. If desired, other suitable current sensing means such as evaporation rate, target temperature, etc., may be employed in place of the transductor sensor.

A schematic circuit diagram of a preferred embodiment of the sensing means 2 4, restart means 114, and switch means 22 is illustrated in FIG. 2. To aid in an understanding of the circuit, a brief outline of its operation Wlll first be given. The current signal, which is sensed by the current sensing resistor 46, is coupled to a trigger circuit 140 through a potentiometer 142. When the current signal reaches a preselected level, as determined by the adjustment of the potentiometer 142, and as explained more fully below, the trigger circuit 140 is triggered, thereby providing a pulse of current at its output. The puse current is amplified by an amplifier 144 and is supplied to a gate 145 of a silicon controlled rectifier 146 to cause the silicon controlled rectifier 146 to fire. When the silicon controlled rectifier 1'46 fires, it couples a capacitor 147 (hereinafter referred to as the de-energizing capacltor) across the primary winding 148 of an input tansformer 150. The de-energizing capacitor 147, which has been previously charged, discharges through the primary winding 148, and the resulting current pulse is coupled through secondary windings 152 of the transformer 150 to a gate controlled switch means 154 in the switch means 22. As a result the switch means 22 is rendered non-conductive and operating current to the electron gun 12 is interrupted. At the same time, the pulse of current, provided by the discharge of the de-energizing capacitor 147 through the conductive silicon controlled rectifier 146 energizes the restart circuit 114. After a predetermined time, the restart circuit 114 applies a current pulse across the input transformer 150 in a direction opposite to that provided by the sensing circuit 24. The current in the opposite direction causes the nonconductive gate controlled switch means 154 to be rendered conductive, i.e., the switch means 22 is closed, and current is again supplied to the electron gun 12.

More specifically, in the illustrated embodiment the current sensing resistor 46 is coupled to the adjustable potentiometer 142 through a normally forward biased diode 164 and a resistance-capacitance input filter 166. A Zener diode 174 is connected across the potentiometer 142, so as to protect the trigger circuit 142 from high voltage transients.

The tap of the potentiometer 142 is coupled to the input of the trigger circuit 140 which, in the illustrated embodiment, is a conventional Schmitt trigger circuit. When the voltage developed at the tap exceeds the triggering voltage of the trigger circuit 140, an output pulse is generated at the output of the trigger circuit. The pulse is amplified by the amplifier 144, which is a transistor connected in a conventional emitter follower circuit. The amplified pulse is then coupled to the gate 145 of the silicon controlled rectifier 146 through a coupling resistor 176 and a Zener diode 178. The Zener diode 178 prevents the silicon controlled rectifier 146 from firing until the pulse exceeds a preselected amplitude. The gate is coupled through a biasing resistor 180 to the negative voltage provided by the power supply 28. The cathode 182 of the silicon controlled rectifier 146 is also connected to the negative voltage.

The anode 184 of the silicon controlled rectifier 146 is biased by coupling the same through a forward biased diode 186 and a voltage dropping resistor 188 to the positive output of power supply 104. In addition, the deenergizing capacitor 147 and the primary winding 148 are both connected in series across the anode-cathode circuit of the silicon controlled rectifier 146. The de-energizing capacitor 147 is thus charged through the blocking diode 186, while the silicon controlled rectifier 146 is non-conductive. Upon firing of the silicon controlled rectifier 146, the de-energizing capacitor 147 discharges through the primary winding 148. The silicon controlled rectifier 146 remains in its conductive state until the current flowing through its anode-cathode circuit is below its holding current, and the signal from the amplifier 144 has disappeared.

The restart circuit 114 is adapted to be energized by the discharge of the de-energizing capacitor 147. The restart circuit 114 supplies a generally pulsating D-C current through the primary winding 148 in an opposite direction to that applied by the de-energizing capacitor 147.

The restart circuit 114 is coupled to the bias supply 104 through the voltage dropping resistor 188, and includes a semiconductor switch 196, which in the illustrated embodiment comprises a four-layer, two-terminal, silicon semiconductor commonly known as a diode thyristor. The semiconductor switch 196 is turned on by a voltage across its terminals in excess of the switching or breakover voltage and is turned off by a reduction in the flow of current through the device to a value below its holding current. The anode 198 of the semiconductor switch 196 is connected through a current swamping resistor 200 and a rheostat 192 to the voltage dropping resistor 188. The cathode 202 of the semiconductor switch 196 is coupled to the primary winding 148 and to the de-energizing capacitor 147. A triggering capacitor 204 is connected across the serially connected semiconductor switch 196, the swamping resistor 200, and the primary winding 148.

Since the capacitor 204 is connected to the bias supply 104, the voltage thereacross exceeds the breakover voltage of the semiconductor switch 196. The capacitor 204 is continually charged by the power supply 104, and when the semiconductor switch 196 is rendered conductive, the capacitor 204 discharges through the switch 196. The semiconductor switch 196 becomes non-conductive when the current supplied by the discharge of the capacitor 204 is reduced to a level below its holding current. The time constant of the charging of the capacitor 204 is adjusted by the settting of the rheostat 192. Consequently, in the illustrated embodiment a generally saw tooth signal, having a predetermined frequency is continually applied across the primary winding 148 in a direction opposite to that provided by the sensing means 24. In addition, a high frequency by-pass capacitor 206 is preferably coupled across the switch 196 to prevent inadvertent turnon as a result of switching transients. A Zener diode 208 is connected across the input of the restart circuit 114, as shown, in order to provide a constant voltage across the circuit and a clamping diode 210 is connected across the trigger capacitor 204.

When the silicon controlled rectifier 146 fires, the charge stored in the de-energizing capacitor 147 applies a high negative voltage to the cathode 202 of the semiconductor switch 196. This causes the switch 196 to turn on and discharge the capacitor 204. The swamping resistor 200, however, reduces this current so that its affect on the current pulse produced by the de-energizing capacitor 147 is minimal. As a result of this premature discharge of the capacitor 204, the normal pattern of the saw tooth waveform being applied across the primary winding 148 is interrupted and a new cycle is instantaneously initiated due to the turn-on of the switch 196. The next saw tooth pulse supplied by the restart circuit 114 to the primary winding 148 restarts the electron gun 12 since it is in an opposite direction to that supplied by the dc-energizing capacitor 147.

To briefly summarize the operation of the sensing and restart circuits, the triggering capacitor 204 together with the semiconductor switch 196 is energized from the bias supply 104 and continually applies the saw tooth waveform across the primary winding 148. When a pulse of current is applied to the gate 145 of the silicon controlled retifier 146 causing it to fire, the previously charged deenergizing capacitor 147 discharges through the silicon controlled rectifier 146 and' supplies a pulse across the primary 148 in a direction from a to b. At the same time, the negative charged side of the de-energizing capacitor 147 applies a high negative voltage to the semiconductor switch 196, causing it to fire and thereby initiate the restarting of the saw tooth waveform being applied across the primary 148. After a predetermined time interval, the charge on the triggering capacitor 204 z is sufficient to cause the semiconductor switch 196 to breakover, thereby pulsing the primary 148 in a direction from b to a.

The restart signal is not only employed to restart the electron gun after it is de-energized by the sensing means 24, but is also employed to render the switch means 22 conductive when power is first applied to the circuit. The restart signal supplied by the restart circuit 114 is coupled by the pair of secondary windings 152 through transformer over-shoot eliminating circuits 212 to the gates 214 of the two gate controlled switches 154. This signal turns on the gate controlled rectifiers 154, or renders them conductive, thereby coupling the voltage regulator 102 to the bias resistor 108. As a result, a positive bias is applied to the grid of the triode 16 and operating power is substantially increased to the electron gun 12.

In the illustrated embodiment, a pair of gate controlled switches 154 are utilized and connected in series relationship with each other between an input conductor 216, connected to the voltage regulator 102, and an output conductor 218 connected to the bias resistor 108, since the voltage rating of a single device may be insufficiently high for certain applications. The switches 154 preferably comprise silicon gate controlled rectifiers, each of which has the gate 214, a cathode 220, and an anode 222.

A pair of gate resistors 224 and 226 are respectively coupled across the gate cathode junction of each silicon gate controlled rectifier 154. Bias for the cathode 220 of the upper silicon gate controlled rectifier 154 and for the anode 222 of the lower silicon gate controlled rectifier 154 is provided by bias resistors 228 and 230 serially connected between the input and output conductors 216 and 218. The junction between the serially connected resistors 228 and 230 is coupled to the junction between the cathode 220 of the upper silicon gate controlled rectifier 154 and the anode 222 of the lower silicon gate controlled rectifier 154. In addition, a transient suppression network 232 is preferably connected across the bias resistors 228 and 230 to protect the silicon gate controlled rectifiers 154. The switch means 22 further includes a pair of input resistor 234 and 236 connected across the primary windings 148.

The turn off of the silicon gate controlled rectifiers 154, as previously mentioned, is effected by coupling a pulse from the de-energizing capacitor 147 across the transformer 150 in a direction from a to b. This signal causes the gates 214 of the silicon gate controlled rectifiers 154 to become negative with respect to the cathodes 220. As a result, the silicon gate controlled rectifiers 154 are rendered non-conductive until a signal from the restart circuit 114 is applied to their gates 214 causing them to become conductive, as previously explained.

The illustrated circuit may be designed to achieve current cutback times as described previously by appropriate selection of component values. The potentiometer 142 is adjusted so that the voltage level at which the Schmitt trigger circuit 140 fires corresponds to a current rise suflicient to indicate that an arc is developing. This current level is preselected to be substantially below the level of current in a steady state are, and for a gun operating normally at 5 amperes, a satisfactory trigger level is about 5.1 to 5.5 amperes. Itv is usually unnecessary to lower the trigger level even if operating at less than 2 amperes, since the current rise time to the trigger level is normally less than a micro-second.

Referring to FIG. 3, the general operational characteristics of the illustrated circuit may be observed. FIG. 3 indicates the variation in voltage across the electron gun versus the current available to the electron gun. The voltage at A represents the open circuit or no-load voltage of the power supply section 28. The line A-B represents the characteristic voltage regulation of the power supply section 28 plus the drop across the triode 16 with constant positive voltage bias applied to the grid 20. The line CD represents the actual regulated voltage as seen by the electron beam gun 12. The slope of the line C-D is reduced from that of the line AB by the regulator loop including the voltage regulator 102. The point D represents the current cutoff or cutback point. This is the point at which the electron gun is drawing the maximum amount of current permitted through the triode control system. The maximum current level is present in the current sensing means 24 and, when it is exceeded, the circuit operates as described above to cut back the current along the dotted line DE to the point B. The point E, as mentioned before, is preferably less than 1 ampere. The operating current may be approximately or of the cutoff point D.

Referring now to FIG. 4, the operation of the circuit may be observed by comparing the current in the electron gun with time. During the interval t t the gun is operating normally at the operating current I At the time t an arc begins to develop and the current increases very rapidly, depending upon the internal impedance of the power supply and the current control device. In high power systems which are stiff, that is, which have low internal impedance, the current may rise at a rate of the order of 10 to amperes per millisecond. The time interval 11-1 is therefore quite small, characteristically only a fraction of a microsecond. When the current reaches the trigger level represented by I at time 1 the current sensing means 24 reacts and causes the switch means 22 to open. The time delay involved in this is represented by the interval 2 4,, and intervals of as short as 10 microseconds are achievable. Once the switch means 22 has opened, and the consequent reverse biasing of the grid 20 in the triode 16 has occurred, the current rapidly falls to the minimum level I within the time interval t t As previously mentioned, the minimum current level I is preferably less than 20% of the operating current level, and highly satisfactory results have been obtained at less than ampere for a five ampere operating level.

As pointed out above, when the time interval t -t is made sufficiently short, the time interval t t may be made significantly shorter than the several thousandths of a second or more typically required for restoration of power after a steady state arc has been allowed to develop. The interval t -t being only a fraction of a microsecond, may be ignored as a practical matter. Accordingly, the circuit components are selected to keep the time interval t -t within the limitations previously set forth.

At the time t the restart circuit 114 closes the switch means 122 and the current rises to the value I restoring full power to the electron gun as indicated by the solid lines in FIG. 4. Occasionally, the arcing conditions may not have completely dissipated at time t and, as a result, the current continues to rise past the level I as indicated by the dotted line. If this occurs, the current sensing means 24 will once again cause current cutback and the time interval t t will correspond generally to the time interval t t The restart circuit will continue to turn the current back on at intervals t -t until the arcing condition has dissipated.

The interval, as mentioned before, may be made very short due to the rapid cutback operation of the circuit. As a practical matter, the minimum possible value is not selected since the shorter the cutback interval, the greater the chances that the arcing condition has not cleared and the circuit will cut back power again. Instead, the interval i 4 is selected to maximize the energy input into the target being heated in the crucible by the electron gun. For high power levels, such as 50 kw. or more per gun, this time interval will typically be about 1 to 10 milliseconds, depending upon the operating conditions. The time constants of the restart circuit are controlled by adjusting the potentiometer 192 so that the charging time of the capacitor 204 will govern the interval r 4 As soon as the capacitor is charged, a pulse in the primary winding 148 will turn the gate controlled switches back on to restore current to the electron gun. The time interval t -t is typically very short, being of the order of less than a microsecond. The delay time of the primary winding 148 and the gate controlled switches 154 must also be considered in setting the time interval 1 4 Referring now to FIG. 5, another embodiment of the invention is illustrated in the form of a block diagram. A stiff high negative D-C supply (not shown), for example 31 kv., 6a, is connected to the terminal 301 which serves as the input terminal for one of the electron beam guns in the system. It is to be understood that the other guns are connected to the D-C power supply with control circuits identical with that illustrated in FIG. 5. The high negative direct current is passed through a high voltage fuse 302 and a vacuum relay 303 and a resistor 304 to the cathode of the current control means 306. The current control means 306 in FIG. 4 is a hard vacuum tetrode capable of conducting very high currents and having very low internal impedance. A satisfactory tetrode for this purpose is a 4CW100,000, available from Eimac Div. of Varian Assoc.

The plate of the tetrode 306 is connected through an RF choke 307 or series of RF chokes to a high voltage discriminator 308. The RF choke 307 is for the purpose of suppressing RF noise in the system and the high voltage discriminator 308 may comprise a tranductor in a manner similar to the transductor 122 in FIG. 1. The output of the transductor or high voltage discriminator 308 is applied to a terminal 309' which, in turn, is connected to the electron gun for supplying high operating current thereto.

A voltage divider including two series resistors 311 and 312 is connected from the juncture between the RF choke 307 and the high voltage discriminator 308 to ground. The voltage across the resistor 312 is fed to feedback signal amplifier 313 and from there to a regulator amplifier 314. A voltage monitor amplifier 316 is also driven by the feedback signal amplifier 313 in order to operate a voltage monitor, not shown, calibrated to provide a continuous reading of the voltage across the electron beam gun. A voltage reference power supply 317 is also connected to the regulator amplifier and the regulator amplifier provides a comparison of the two voltages and the difference signal is supplied as an output. The output signal is fed through a photon coupler 318 to a photon coupler booster amplifier 319 operated from a power supply 321. A transistor 322 has its base connected to the output of the booster amplifier 319 and operates in a manner similar to that of the switch means 22 in the circuit of FIG. 1. The transistor 322 is an NPN transistor having its emitter connected to the negative side of a grid bias power supply 323. The grid bias power supply may be, for example, a power supply supplying 400 volts regulated by Zener diodes and isolated from the high voltage of the system. The positive side of the grid bias power supply 323 is connected to the cathode of the tetrode 306. A biasing resistor 324 is connected across the cathode and grid of the tetrode 306.

The voltage across the electron gun is sensed by the voltage divider 311, 312 and fed back thorugh the signal amplifier 313. The regulator amplifier 314 compares this voltage with the reference power supply voltage 317 and provides an adjustment signal through the photon coupler 318 (for isolating the control elements from the high voltage elements) to the photon coupler booster amplifier 319. The output of the booster amplifier 319 is applied to the base of the transistor 322 to regulate the conduction of the transistor and thereby adjust the potential on the grid of the tetrode 306 to regulate the voltage across the electron beam gun.

The tetrode 306 is provided with a screen power supply 326, the positive side of which is connected to the screen of the tetrode and the negative side of which is connected to the cathode. The screen power supply 326 is preferably Zener regulated and high voltage isolated, and maintains the screen voltage at a suitable level above the cathode voltage, for example 500 volts. By utilizing a tetrode 306 rather than a triode as in the circuit of FIG. 1, the circuit of FIG. 4 may be more suitable for very high voltage applications such as those exceeding 50 kw. The presence of a screen provides very high gain in the tetrode and enables regulation of the gun current by relatively smaller variations in grid bias. Were a triode used in place of the tetrode 306, provision would have 0t be made for varying the grid bias over a much larger range than is necessary with a tetrode.

The resistor 304, which is in series with the electron beam gun, is used for sensing the gun current in a manner similar to the use of the resistor 46 in the embodiment of FIG. 1. The voltage across the resistor 304 is sensed by the current cutback circuit 327, which is supplied by the amplifier power supply 321. When the voltage drop across the sensing resistor 304 exceeds the trigger level, the current cutback circuit 327 drives the transistor 322 into full conduction, placing the full negative bias on the grid from the grid bias power supply 323. This cuts off the tetrode 306 and prevents current from reaching the electron beam gun. The current cut back circuit 327 then operates, after a predetermined time delay, to reduce the level of conduction of the transistor 322 and thus reduce the back bias applied to the grid of the tetrode 306 when the gun current is to be turned back on'.

It is preferable that the control circuitry, eg the amplifier 321, the cutback circuit 327, the amplifier 319, and the power supplies 323 and 326 be isolated from the high voltage components of the system. As previously mentioned, the photon coupler 318 is employed for this purpose as to the amplifier 319. Other suitable techniques may be employed for the other control components.

The timing sequence of the operation of the circuit illustrated in FIG. 4 is generally the same as the sequence previously described in connection with the circuit of FIG. 1. The advantage of the circuit of FIG. 4 is that it is particularly useful in connection with very high power level systems. For example, a system of the type illustrated in FIG. 4 may be utilized for providing as much as 200 kw. of power per gun. The high gain of the tetrode 306 facilitates voltage regulation, and the use of a strong back bias for cutting off the tetrode ensures complete cutoff and fast overall response.

Thus, a novel and improved method and apparatus has been provided which regulates the voltage developed across the electron gun, and which permits the current to be selectively adjusted between preselected limits and 15 regulated at desired levels. At the same time a protective feature is provided to cut back the current supplied to the electron gun so as to preclude the occurrence of damaging effects which may occur due to arcing, and to facilitate rapid restoration of power. In addition, the invention makes possible the energization of a plurality of parallel connected electron guns and the limiting of the current supplied to a particular gun during arcing without affecting the current supplied to the other guns.

Various changes and modifications may be made in the above described method and apparatus without deviating from the spirit or scope of the present invention. Various features of the present invention are set forth in the accompanying claims.

What is claimed is:

1. A method for regulating the power of an electron gun employed in an electron beam furnace system wherein the gun current is susceptible of rising, upon the occurrence of an arc, from an operating level to a higher steady state level which is at least three times the operating level in less than about one millisecond, said method comprising: sensing a rise in the current supplied to the electron gun to a trigger level which is substantially lower than the higher steady state level, reducing the supplied current to a level sufficient to starve the arc before the gun current can rise to the higher steady state level, and subsequently restoring current to the electron gun.

2. A method according to claim 1 wherein the cut back time interval is of the order of microseconds.

3. A method for regulating the power of an electron gun employed in an electron beam furnace system wherein the gun current is susceptible of rising, upon the occurrence of an are, from an operating level to a higher steady state level which is at least three times the operating level in less than about 1 millisecond, said method comprising: sensing a rise in the current supplied to the electron gun to a trigger level which is substantially lower than the higher steady state level, reducing the supply current to a level sufficient to starve the are within a period of time which is less than 1 millisecond from the time the current reaches the trigger level, and subsequently restoring-current to the electron gun.

4. A method according to claim 3 wherein the current flow to the electron beam gun is restored to its original level in less than 10 milliseconds.

5. A method according to claim 1 wherein the cut back time interval is less than 100 microseconds.

'6. Apparatus for regulating a power of an electron gun employed in an electron beam furnace system wherein the gun current is susceptible of rising, upon the occurrence of an arc, from an operating level to a higher steady state level which is at least three times the operating level in less than about 1 millisecond, including in combination, means for sensing a rise in the current supplied to the electron gun to a trigger level which is substantially lower than the higher steady state level, a current control device coupled in series with the electron gun, and circuit means connecting said current control de vice to said sensing means for reducing current flow to the electron gun, upon a rise in current to the electron gun due to an arc, to a level sufiicient to starve the are before the gun current can rise to the higher steady state level, and for subsequently restoring current to the elec tron gun.

7. Apparatus according to claim 6 including voltage regulating means responsive to the voltage developed across the electron gun during normal operation of the electron gun to maintain the voltage developed across the electron gun substantially constant.

8. Apparatus according to claim 6 wherein said current control device comprises a vacuum tube having its anode-cathode circuit connected in series relationship with 16 the electron gun, and having a grid connected to said circuit means.

9. Apparatus according to claim 8 wherein said vacuum tube comprises a triode.

10. Apparatus according to claim 8 wherein said vacuum tube comprises a tetrode, and including means for placing a bias voltage on the screen of said tetrode which is substantially higher than the cathode voltage.

11. Apparatus according to claim 8- wherein said circuit means include first switch means closed during normal operation of said electron gun, means responsive to the opening of said first switch means for providing bias to said grid to reduce conduction of said vacuum tube, and means coupling said sensing means to said first switch means to open said first switch means when the current rises to said preselected level.

12. Apparatus according to claim 11 wherein said first switch means include means therein for maintaining said first switch means in its non-conductive state, and wherein a selectively energizable restart circuit is coupled to said trigger means and said first switch means for rendering said first switch means conductive after a predetermined delay, said restart circuit being energized by output signals from said trigger means.

13. Apparatus according to claim 11 wherein said sensing means include a resistor connected in series with said electron gun, a selectively energized trigger circuit coupled to said resistor which provides an output pulse when said current exceeds a predetermined level, selectively energizable second switch means coupled to said trigger circuit, said second switch means being rendered conductive by the pulse from said trigger circuit, and means responsive to the conductive state of said second switch means for supplying a signal to said first switch means to thereby open said first switch means.

14. Apparatus according to claim 13 wherein said selectively energizable second switch means include a silicon controlled rectified fired by the pulse from said trigger circuit, and wherein said first switch means include at least one gate controlled switch which is rendered nonconductive by the firing of said silicon controlled rectifier.

15. Apparatus for regulating the power of an electron gun employed in an electron beam furnace system wherein the gun current is susceptible of rising upon the occurrence of an arc, comprising, a source of direct current at a high negative potential with respect to a reference potential, a vacuum tube for connecting said source to said electron gun, said vacuum tube having a cathode connected to said source and having an anode connected to the electron gun, means for sensing the current sup plied to the electron gun and immediately providing a signal when said current rises above a preselected level, said vacuum tube having a grid for controlling the conduction thereof, and circuit means connected to said grid and to said sensing means, said circuit means being responsive to said signal to immediately bias said grid to reduce current flow to the electron gun to a level sufiicient to starve the arc, and to remove the bias to restore current to the electron gun a predetermined time after the occurrence of said signal.

References Cited UNITED STATES PATENTS 3,147,400 9/1964 McClay 307260 DONALD D. FORRER, Primary Examiner B. P. DAVIS, Assistant Examiner US. Cl. X.R. 

